Stereophonic sound reproduction systems employ psychoacoustic effects to provide a listener with the impression of a multiplicity of separate real sound sources, for example musical instruments and voices, positioned at several distinct locations across the space between the left and right loudspeakers which are usually placed symmetrically to either side in front of the listener.
Pairwise mixing is an example of an early technique for producing such an impression. The sound is provided to both channels in phase, with an amplitude ratio following a sine-cosine curve as a sound source is panned from one side of the listener to the other. While this approach has been a generally accepted one, it has proved deficient in several ways; the apparent location of the sound is not stable when the listener's head moves, and sounds between the loudspeakers appear to be above the line joining them. More recent research in psychoacoustics has shown that when sound is diffracted round the listener's head, in general the left and right ears hear different transfer functions applied to the sound; an impulse will reach the far ear later than the near ear, and the shadowing provided by the head will alter the amplitude of the sound reaching the far ear relative to that reaching the near ear, the amplitude differences being a complicated function of frequency. These functions are termed "head-related transfer functions" and include effects due to reflections of sound by the pinnae and torso of the individual listener.
A somewhat simplified model of the head as a sphere, with orifices at left and right representing the ears and without the equivalent of pinnae, can be used to derive a generic HRTF theoretically or through numerical analysis. Because there are no pinnae, there is no difference between the HRTFs for sounds to the front of or equally to the rear of the lateral center line. Also, the lack of pinnae and torso modifications precludes differences due to the height of the sound source above the plane containing the ears. Nevertheless, the "spherical head" model has at least pointed the way to understanding the subtleties of HRTF effects.
An alternative reproduction method to stereophony is binaural recording, which typically employs a "dummy head" or manikin of a generic character, with pinnae and torso effects included, which has HRTFs that may be considered "average." Microphones are placed in the ear canals of the dummy head to record the sound, which is then reproduced in the listener's ears using headphones. Because individuals differ in head size, placement and size of the ears, etc., each listener would obtain the most realistic binaural reproduction if the dummy head used for recording were an exact replica of his own head. The differences are sufficient that some listeners may have difficulty in differentiating the front or rear locations of some sounds reproduced this way. A further disadvantage of this method is that when reproduced over loudspeakers, sounds intended for reproduction only in the left or right ear are heard differentially by both ears, and the HRTFs corresponding to the loudspeaker locations are superimposed onto the sounds, contributing to unnatural frequency response effects.
Various methods for cancellation of the crosstalk between the loudspeakers have been devised, and this art is assumed in this patent application. Thus, the reproduction of binaurally recorded sound could take place either on headphones or through loudspeakers with the crosstalk cancellation method applied in the latter case.
In order to produce realistic recording and reproduction of sounds in specific locations relative to the listener, it is desirable to have a method which can simulate any location of a monaural source within the sound stage reproduced through a pair of loudspeakers. Since pairwise mixing has been found to have considerable drawbacks, a method that employs the known psychoacoustical effects of HRTFs is significantly better. Furthermore, such methods can also simulate sound locations to the sides and rear of the listener.
Although digital filtering can be used to provide these complex enhancements of the sound signals prior to mixing down onto two-channel media, for reproduction on a pair of loudspeakers, the cost and complexity of such filtering is often an obstacle to obtaining the most realistic reproduction. Therefore, the efficiency of the method must also be considered, as a method using fewer coefficients to obtain the same result will typically be lower in cost.